where $\mu$, $\lambda_L$ and $K$ (bulk modulus) are material parameters.
$\mathcal{L}^{-1}$ is the inverse of the Langevin function, $J = \mathrm{det}\mathbf{F}$,
$\bar{\mathbf{C}} = J^{-2/3} \mathbf{F}^t \mathbf{F}$ and $\bar\lambda$ is a measure of the network stretch.